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926 CHAPTER 36 1958 Guide cause of the high vacuum, a small amount of the water introduced in the evaporator is flashed into steam. As this requires heat, and the only source of heat is the rest of the water in the evaporator tank, this other water is almost instantly cooled to a temperature corresponding to the boiling point determined by the vacuum maintained. The amount of water flashed into steam is a small percentage of the total water circulated through the evaporator, amounting to approximately 11 lb per (hr) (ton) of refrigeration developed. The remainder of the water at the desired low temperature is pumped out of the evaporator and used at the point where it is required. The ejector compresses the vapor which has been flashed in the evapo rator, plus any entrained air taken from the circulated water, to a some what higher absolute pressure. The vapor and air mix with the impelling steam on the discharge side of the jet, and the total mixture then passes from the ejector into the condenser. The slight amount of air which may be entrained in the cooled water is removed by a small secondary ejector which raises the pressure sufficiently so that the air can be discharged to the atmosphere. A secondary con denser is then necessary to condense the* steam in the secondary jet. While a single booster of smaller than 15 tons capacity is difficult to build, steam jet vacuum cooling units have been built for as small as 5 to 6 tons capacity. They can readily be built for steam pressures of from 5 to 200 psig, and condenser water temperatures as high as 90 F. The steam consumption in pounds per hour per ton of refrigeration increases rapidly as the booster steam pressure is lowered. For example, the lower ing of the booster steam pressure from 200 to 90 psig results in an increase in steam consumption of approximately 5 percent, whereas a further decrease in booster steam pressure to 10 psig increases the steam consump tion by approximately 72 percent over that required at 200 psig. The capacity of a steam jet system is usually controlled by controlling the number of boosters in use since the unit usually has several boosters operating on the same evaporator. Usually one booster is automatically controlled, whereas the others are manually operated. The capacity is dependent, as for all compressors, upon the evaporator temperature, or in other words, the suction pressure. For example, the capacity is lowered approximately 17 percent if the evaporator or chilled water temperature is lowered from 50 to 45 F. The capacity therefore can be controlled to some extent by regulating the evaporator temperature. The Absorption System The absorption and compression refrigeration cycles differ only with respect to the method of compression. Each cycle requires a condenser, expansion valve, and evaporator, but the absorption cycle utilizes three major equipments in place of the mechanical compressor; these equipments are the absorber, the pump, and the generator. Vapor from the evaporator is absorbed by a low temperature absorbent fluid which is then pumped to the generator where heat is supplied to boil off the refrigerant. The ab sorbent is now cooled and readmitted, through a pressure-reducing valve, to the absorber. In addition to the three primary equipments of the absorption cycle it is necessary to provide auxiliary equipment, usually an analyzer and a rectifier, to remove from the refrigerant leaving the generator, insofar as is possible, the absorbent which vaporizes and leaves the generator with Refrigeration 927 the refrigerant. Removal of this material is of great importance to effec tive operation of the system, since even a small concentration of absorbent in the refrigerant will suffice to reduce greatly the evaporator pressure required for maintenance of a given evaporator temperature. Thermo dynamic analysis of absorption cycles is relatively complex, and requires the use either of tables or graphs showing the equilibrium relationships and thermodynamic properties of the refrigerant-absorbent combination; Data of this kind are given in the first .book in the bibliography. A discus sion of various absorbents is given in the sixth book. Thermodynamically, the effectiveness of a refrigerant-absorbent combination increases directly with its negative deviation from .Raoult's Law. Fig. 7 shows a typical absorption cycle flow diagram. Cooling water first goes through the absorber (where it extracts the heat of absorption which is liberated by the refrigerant vapor as it goes into solution), then through the condenser, and .finally through the rectifier. Refrigerant Wa(e> inlet pump Fig. 7. Closed Absorption System from the evaporator enters the absorber where it goes into solution in the absorbent; the high concentration solution is then pumped to the generator where heat is supplied; the refrigerant (with some absorbent vapor) leaves for the rectifier and the warm low concentration solution is returned to the absorber. In the rectifier selective condensation occurs, the concen tration of the absorbent in the condensate being much greater than its concentration in the entering vapor mixture; rectifier condensate is dripped back to the generator. The ratio of refrigerating effect to heat input (the performance ratio or commonly used efficiency measure of absorption machines) is only 40 to 45 percent with the ordinary ammonia absorption system and, aside from the inherent disadvantages involved in the use of a toxic and explosive refrigerant, this is not sufficiently high to make it competitive with other types of systems when used in air conditioning applications. Therefore, recently, several absorption systems using hygroscopic brines of salts such as lithium chloride or lithium bromide3 (solids in the pure state) as absorb ents and water as the refrigerant have been developed. Such systems are to higher temperature applications but, thermodynamically, have the advantages of a refrigerant with a high latent heat of vaporization and nonvolatile absorbents with a large negative deviation from Raoult's Law.